The stenosis induced by cardiovascular and cerebrovascular lesion is one of the most important reasons of complications of many fatal cardiovascular and cerebrovascular diseases. Metal vascular stent implantation is one of the main methods for treating stenotic cardiovascular and cerebrovascular diseases. In recent years, the magnesium alloy has become a research hotspot of cardiovascular stent biomaterials due to its excellent mechanical properties and biodegradable properties, and non-toxicity of its degradation products to the human body.
However, the magnesium alloy is rapidly degraded under physiological conditions due to its active chemical property, so that excess hydrogen is easily generated around the implanted tissues and a local alkaline increase in surrounding tissues and the accumulation of the secondary corrosion products are caused, resulting in premature loss of mechanical properties, poor blood compatibility and cell compatibility of the material, and toxic reactions to surrounding tissues, eventually leading to delayed healing of tissues and even implantation failure. The corrosion behavior and biocompatibility of the material are closely related to the surface properties of the material, therefore, the electrochemical degradation behavior of the magnesium alloy can be regulated, the blood compatibility can be improved and the endothelial tissue healing can be promoted through surface modification of the material as a vascular stent material, which is of great significance to its clinical application.
In view of the problem of too rapid degradation of the magnesium alloy in a physiological environment, the corrosion resistance of the magnesium alloy is mainly improved from both aspects of alloying and surface modification. The alloying can significantly improve the mechanical properties of the magnesium alloy, but the corrosion resistance of the prepared magnesium alloy in the complex physiological environment remains to be improved, and most of the alloying elements cannot effectively improve the biocompatibility of the material. Therefore, the surface of the magnesium alloy prepared by alloying is usually lack of biological activity.
At present, the research on improving the corrosion resistance of the magnesium alloy through surface modification mainly focuses on three aspects. One is to form a chemical conversion layer on the surface by surface chemical treatment or electrochemical treatment; the other is to form a surface modified layer by changing the microstructure of the surface; the third is to form a surface covering layer on the surface of magnesium alloy by introducing organic molecules and macromolecules, or preparing inorganic non-metallic coatings on the surface. The conversion layer or covering layer with better corrosion resistance formed on the surface through these methods can isolate a matrix from the surrounding medium, thereby effectively improving the corrosion resistance of the magnesium alloy and significantly reducing the physiological side reactions caused by rapid degradation, and thus improving the biocompatibility of the material to a certain degree.
The introduction of bioactive molecules on the surface is one of the most effective methods for improving the biocompatibility of the magnesium alloy and other biological materials. However, the surface modification strategy for non-degradable biomaterials often needs to be performed in electrolyte solutions, so that the corrosion degradation of magnesium alloy may be caused due to the active chemical property of the magnesium alloy, and thus the corrosion resistance of the magnesium alloy should be firstly improved before using these strategies. Although a great deal of effective work has been performed on the surface modification of the magnesium alloy, the current surface modification method of the magnesium alloy as an intravascular implant material does not achieve clinically satisfactory results in both aspects of improving the corrosion resistance and enhancing the biocompatibility. The surface layer constructed by surface chemical polymerization, self-assembly surface modification, and surface in-situ biological molecule immobilization technology and so on is thinner, has fewer biomolecules, and is limited in the improvement on the corrosion resistance and biocompatibility of the magnesium alloy. In the degradation process of the material, the surface biomolecules are firstly degraded and lost, and the magnesium alloy will soon lose its biological activity; various macromolecules or ceramic coatings have significant effects in improving the corrosion resistance of magnesium alloy, but may still cause blood coagulation and delayed healing of endothelium when used as an intravascular implant material.